A localized surface plasmon resonance (LSPR) can be induced in an electroconductive fine structure. Resonance conditions of the LSPR depend on the refractive index and dielectric constant surrounding the electroconductive structure. Therefore, a change in the dielectric constant around the electroconductive structure can be detected by a change in the resonance conditions. The change in the resonance conditions can be detected by measuring a change in the optical spectrum of the light beam projected to and transmitted through the electroconductive structure.
The LSPR is sensitive to a change in the refractive index and dielectric constant of the medium surrounding the electroconductive structure, and is applicable to high-sensitive detection of a refractive index.
As shown below, when a biological reaction causes a change in the dielectric constant, this change can be utilized for a high-sensitive bio-sensing. Therefore, the LSPR is promising in broad application fields including medical treatment, foodstuffs, and environment.
For example, occurrence of an antigen-antibody reaction around the electroconductive structure can be detected by utilizing the LSPR. Richard P. Van Duyne et al. (NANO LETTERS 2004, vol. 4, No. 6, 1029-1034) discloses a silver microparticulate thin layer structure formed as an electroconductive structure on a smooth base plate. With this structure, the antigen concentration is determined from a change in the optical spectrum between a state of antibody adhesion and a state of additional adhesion of antigen around a silver microparticulate thin film structure.
In other examples, enzyme-substrate complex formation, complementary base pair formation by DNA hybridization, and so forth can be detected similarly.
U.S. Patent Application Publication No. 2007/0263221 discloses a sensor, which has a first reflector, a translucent body, and a second reflector placed in the named order from the measurement light-input side. In this sensor, the light beam introduced through the first reflector into the translucent body is repeatedly reflected between the first reflector and the second reflector to cause multiple reflection and multiple interference. Adhesion of a sample to the sensor changes the absorption peak spectrum by the multiple interference, enabling analysis of the sample. In addition to the above absorption peak spectrum given by the multiple interference, an absorption peak is developed by the localized surface plasmon resonance, and this change in the absorption peak spectrum enables analysis of the sample. U.S. Patent Application Publication No. 2007/0263221 describes a structure of the first reflector having fine holes and a structure having metal fine particles, as the first reflector.
The above prior art techniques are not sufficient in the measurement sensitivity for high-sensitive detection of a low concentration of a target substance.
For high-sensitive detection of a target substance, it is necessary to increase the wavelength shift of the resonance spectrum by adhesion of the target substance and to decrease the width of the resonance spectrum. For example, in detection of a target substance adhesion by difference of the spectra caused by a reaction, a small width of the resonance spectrum and large shift of the wavelength will increase the differential of the spectrum by the reaction to enable high-sensitive detection. For this purpose, U.S. Patent Application Publication No. 2007/0263221 proposes a structure which has a metal hole-array structure or a metal microparticulate arrangement structure as the reflector of the resonator. However, the proposed structures do not achieve any of the effects of decrease of the width of the resonance spectrum and increase of the shift of the resonance spectrum.